200 ALASKAN GLACIER STUDIES
and form, according to the conditions surrounding their accumulation and the length of time occupied by it. One notable difference from normal valley glacier deposits would be found in some of the valleys, in which a sheet of somewhat weathered, angular fragments would overspread the ground moraine. This, representing the ablation moraine, would form a thin sheet, varying in thickness and with a somewhat hummocky topography.
The Through Glacier. The through glacier condition has already been described with such fulness that little remains to be added here. It is a common condition in the Alaskan coast ranges to find mountain valleys so flooded with ice to the divides that one can travel by fairly even grade from the dissipator of a glacier up across a broad, flat divide-reservoir, then down to a dissipator in another valley, without being able to determine where the boundary line between the two glaciers is to be drawn. Such is the through glacier, occupying a through valley with low divide. In some cases more than two ice tongues extend from, a through glacier divide area. Because of the extensive development of the through glacier condition there is a network of ice-flooded valleys, the full extent of which can only be inferred in the present state of our knowledge of the Alaskan coast ranges back from the coast.
The through glacier condition is certainly not confined to the Alaskan mountains, but it is more strikingly developed there than in any other region of which we have seen description. Its extensive development here is dependent upon the combination of three very favorable conditions, as follows:—(1) the fact that at a former period the glacier systems of the Alaskan coast were far more extensive than now; (2) the fact that this period of glacier expansion lasted long enough for extensive glacial erosion to lower many divides; and (8) the fact that, even in the present shrunken condition of the glaciers the snowfall is so heavy that both glacially-deepened valleys and lowered divides are still ice flooded. There is reason to believe that in the former period of expanded glaciers, and before the present valleys were so deepened by glacial erosion, ice currents streamed across many divides, and in different directions from now. Some of these divides were completely removed by erosion, but many were only partly removed; and in the latter case, as the volume of the ancient glaciers shrank, the time came when some of these lowered divide areas became the flat ice-divides of through glaciers. Doubtless in other cases, back among the mountains where, as Russell vividly describes it, the ice-flooded valley condition is so well developed, both valleys and divides are still so deeply buried beneath snow and ice that glaciers are streaming across divide areas and still lowering divides between mountain valleys.
One of the needs of Alaskan glacier study is the exploration and mapping of some of the through glacier systems, for at present we know very little about the conditions a few miles back from their termini. It will be interesting to know more about the source of the ice supply, though in all probability this varies greatly in different glaciers. Lying, as they do, in broad valleys among extensive, lofty, snow-covered mountains, it is certain that they receive important avalanche contributions from the enclosing mountains, as well as contributions from tributaries descending to them from the upper mountain valleys. In the larger, more active, through glaciers, these must furnish the main ice, supply; but in some of the smaller, less active glaciers, like the Orange Glacier, and perhaps even the south arm of the Nunatak Glacier, a large part of the snow supply, and perhaps even the dominant part of it, apparently comes from the snow that falls on the broad divide and upon the glacier surface itself.